Indoor air quality is a term that describes the cleanliness or livability of air inside a structure where people live, work, or play. Particulates are a major component of indoor air quality, and the size and quantity of airborne particulates are key indicators of indoor air quality. The mixture of particles inside a building can include many sizes of particles from many origins such as: plant and animal allergens, mold/mildew allergens, insect allergens, outdoor air contaminants, tobacco smoke and a wide variety of other materials. Sufficiently small particles, those less than 10 microns in diameter, can stay airborne for extended periods. Larger particles are only airborne when disturbed by airflow or human activity such as cleaning. It is a primary objective in particulate measurement investigations to identify the distribution of the particle count by size, and then use the size information to further define the possible origin of particles.
Size distribution can be as simple as large and smaller particles. Existing EPA regulations have explicit limits on particulate matter at 10 micron and 2.5 micron size ranges. Particles with a diameter of 2.5 microns are called fine particulates. For example, a basic separation of particulates around the regulatory-based 2.5 micron cut point allows for the identification of larger “dust” and smaller “fine” particles. The time varying relationship between these two measurements can provide a distinct advantage in identifying the source of particulate problems inside a structure.
A major problem confronting the diagnosis of indoor air quality problems and particulate measurement is the constant change common in the indoor environment. The individual constituents of the indoor aerosol vary continuously due to many reasons such as; changes in the climate or weather, proximity to major outdoor pollution sources or human activity inside the building. Hence, the government mandated fine particulate level of 2.5 microns may not be sufficient or the right way to separate particles based on size in every case. Depending on the conditions mentioned earlier, it might make sense to look for particles over 10 microns only. Or in other cases it is necessary to look for particles between 1 and 10 microns such as mold, mildew and chemical weapon spores. Regardless of the reason, the ability to change the particle sizing criteria based on various initiating events will be a substantial refinement in particulate detection systems and will be a major improvement over existing art.
Light scattering photometers, usually calibrated against a known standard of particulate such as Arizona road dust, provide an estimation of total suspended particulate matter in the air. These instruments are sensitive to particle sizes (diameter) from 0.1 micrometer to around 10 micrometers. The major advantage of these instruments is their relative low cost, ease of use, and portability. The major disadvantage of these instruments is their inability to separate particles based on size.
Laser particle counters are similar to photometers, however they normally come equipped with particle sizing or other physical means to remove or screen out particles of a certain size. Particle sizing or selection of the “cut point” is done manually to the instrument prior to putting the instrument in use. There is no method for dynamically changing the size selection. These instruments are also easily overwhelmed with high particulate levels typical in second hand tobacco smoke.
Dynamic particle sizers are also available. These instruments use more complex methods than light scattering to determine the composition and size of aerosols. While able to output the quantity of particles in specific size brackets, the major limitations of these instruments are their inability to handle large quantities of particles commonly found in indoor environments, the need to manually adjust the specific cut point of the particle sizer, and the need to recalibrate the instrument in a laboratory environment before it can return to service in the field.